Steam version crashes in Snowy Valley

[vivicidal]

I just bought Penumbra: Overture from Steam. I tried the demo a few months ago and liked the game so I finally decided to buy it, but now the game works until I leave the ship and get to the snowy valley. I see the snow for a few seconds and as the screen gets brighter, the whole game crashes.
I get an error talking about ntdll.dll, and the whole game is closed. I am so dissappointed - I've bought the game and now I get less far than I got on the demo a few months ago!

I am running Vista with a core 2 duo E7200, Nvidia 8600GT, 2GB RAM, MSI P35 Neo-F etc etc, and my graphics drivers are up to date.

I verified my files on Steam, but the issure hasn't gone away. Any ideas will be gratefully received.